Thermochemically treated oligomeric and/or polymeric derived silicon carbide fibers

ABSTRACT

Silicon carbide fibers which are derived from oligomeric and/or polymeric precursors are modified and strengthened in annealing the silicon carbide fiber at temperatures in excess of 800° C. under a nitrogen atmosphere in the presence of carbon particles. The modified fibers can be used to make ceramic, metal, and plastsic composites.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a thermochemically treated silicon carbidefiber derived from organosilicon oligomeric and/or polymeric precursorsincluding doped oligomers and polymers. The invention also relates tothe process of thermochemically treating the fibers by annealing theoligomeric and/or polymeric derived silicon carbide fibers in an excessof carbon particles under an interactive gas atmosphere such as nitrogengas.

2. Description of Related Art and Problems in the Art

Composites are structural materials formed by the combination of severalmaterials. Composites are used where there is a need for a lighterweight structure which must be stronger, tougher, and able to surviveharsher environmental and temperature cycling conditions which cannot bemet by traditional unreinforced single materials or alloys. Compositesprovide the opportunity to design and fabricate structural materialhaving specific chemical, thermal, mechanical, and electromagneticproperties. Composites can be fabricated from plastic, ceramic and/ormetal materials.

Flaws of any size are likely to cause failure of loaded brittle ceramicor metal materials. This flaw intolerance tends to limit the use ofbrittle materials, particularly ceramics, in situations which requirestructural reliability.

Ceramic materials can be strengthened by the addition of fibers. Thefibers may be composed of many materials depending on the particularcomposite and fabrication and operating conditions for that composite.Ceramic and metal composites can be strengthened by the addition ofceramic fibers formed by any of the very well known means or purchasedfrom commercial sources.

Theoretically, the appropriate inclusion of fibers in a ceramic matrix,particularly high strength ceramic fibers, should increase the fracturetoughness and strength of the ceramic product. In fact, it has beennoted that the performance of ceramic fiber reinforced materials hasfallen short of expectations.

Ceramic and metal composites are formed by hot pressing, rolling,molding, sintering, and other techniques in which the compositesmaterials are subjected to high temperatures and pressures. Theseconditions tend to degrade the strength of known ceramic fibermaterials, particularly those derived from organosilicon oligomericand/or polymeric precursors. Both chemical and mechanical reasons havebeen identified for the poor results of oligomeric and/or polymericderived reinforcing fibers. In some cases, chemical incompatibilitybetween the fiber and the matrix results in such a poor bond between thematrix and the fiber that no load can be transferred to the fiber. Inother cases the fiber and matrix react together causing too strong of abond to form at the fiber/matrix interface. The fiber may also losestrength during the various densification processes used to form thecomposite. These changes in the fiber and the matrix ultimately lead toa weakening of the composite material as a whole.

The thermochemical instability of fibers derived from organosiliconoligomer and polymer precursors such as silicon carbide are a majorlimitation in the choice of processing conditions and matrix materialsfor the end product composite. The exact reasons for the degradation of,for example, oligomer and polymer-derived silicon carbide fibers is notcompletely understood. Degradation products such as silicon dioxide andcarbon dioxide have been identified being exuded by the fibers. Someauthors, such as Anderson and Warren, in "Silicon Carbide Fibers andTheir Potential Use in Composite Materials, Part I Composites," Vol. 15(No. 1) p. 16-24 (1984), propose crystallite growth, micro-porosityformation and (flaw) growth as possible reasons for fiber degradation.

Fiber surface coatings and fiber surface treatments have been proposedas means to prevent fiber degradation and to make the fiber morechemically compatible with the matrix. In U.S. Pat. No. 4,340,636 Deboltet al. suggest vapor phase treating a stoichiometric CVD silicon carbidefilament to create a carbon rich silicon carbide surface which is morechemically compatible with organic polymers and alumina materials whenforming composites with those materials.

Bender et al. have shown that a coating of boron nitride (BN) on thesurface of a polymer-derived silicon carbide fiber strengthened thefiber matrix composite and they showed that the coating acted as adiffusion barrier to protect the fiber from oxidation or fromvolatilizaton from reactions with the matrix, Bender et al. "Effect ofFiber Coatings and Composite Processing on Property of Zirconia-BasedMatrix Silicon Carbide Composites," American Ceramic Society Bulletin,Vol. 65, No. 2, Feb. 1986, p. 363-369.

Others have also suggested the use of boron nitride as a coating tostrengthen ceramic fiber composites. Singh and Bruhn "Effect of BoronNitride Coating on Fiber-Matrix Interactions" Ceram. Eng. Sci. Proc., 8(7-8) pp. 636-643 (1987); Rice et al., "The Effect of Ceramic FiberCoating on the Room Temperature Mechanical Behavior of Ceramic-FiberComposites", Ceram. Eng. & Sci. Proc., 8th Annual Conference,July-August 1984, published by the American Ceramic Society, Columbus,Ohio, 1984; Lewis and Rice, "Further Assessment of Ceramic CoatingEffects on Ceramic Fiber Composite", NASA Conf. Publ. 2406, Proceedingsof a Joint NASA/DOD Conference, Jan. 23-25 1985.

SUMMARY OF THE INVENTION

In accordance with this invention, it has been found that modifiedfibers useful for inclusion in composite matrices, particularly metaland ceramic matrices, can be produced by annealing an organosiliconoligomer and/or polymer derived-silicon carbide fiber under aninteractive gas atmosphere in the presence of carbon particles. Thefibers of this invention have greater stability, greater flexibility,greater ability to retain physical and chemical properties at elevatedtemperatures and greater ability to retain these properties in thepresence of aggressive environments.

Accordingly, the primary object of this invention is to produce a morestable surface-modified ceramic fiber derived from oligomeric and/orpolymeric sources for use in composite material.

A further object of this invention is to provide a simple andinexpensive process for producing surface modified ceramic fibers foruse in composite matrices.

Other objects, advantages, and novel features of the invention willbecome apparent from the following detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission election micrograph (TEM) (A) Bright field (BF)and B) selected area diffraction pattern (SADP) of an untreated fibershowing the amorphous structure of the fiber.

FIG. 2 A) and B) are scanning election micrographs and C) is atransmission electron micrograph (TEM) bright field (BF) of a treatedfiber of this invention showing the substantially crystalline alphasilicon carbide outer zone and a microcrystalline beta silicon carbideinner core.

FIG. 3 is a graph comparing the strength of fibers treated under theconditions of this invention to the same fibers treated under differentconditions.

A more complete appreciation of the invention and many of the attendantadvantages of the invention will be readily obtained and will becomebetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings, wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The surface modified fibers of this invention find use in compositematrices. The fibers of this invention are ceramic fibers derived fromorganosilicon oligomeric and/or polymeric precursers such aspolycarbosilane or other silicon-carbon-hydrogen based polymers. Theseoligomer and polymer precursors can include other inorganic materialssuch as metals including zirconium, tungsten, titanium and the like. Thetechniques for the preparation of these ceramic fibers are well knownand are described in U.S. Pat. No. 4,100,233 and European PatentApplication 0,055,076 published Jun. 30, 1982. Included within thisinvention are strengthened fibers derived from all oligomeric andpolymeric organosilicon precursors including those oligomers andpolymers which are substituted with a metal group. Those polymers andoligomers containing metals will be referred to generally as doped. Theterm oligomeric/polymeric-derived fibers is intended to include thoseprecursors which are doped as well as those which are not.

Generally, the strengthened fibers of this invention are made by firstcutting the pyrolyzed fiber into appropriate length for use incomposites. These lengths can be any length with practical limitationsimposed by the size of the heat treatment furnace. Usually the fibersare between 50 and 100 millimeters in length and most conveniently are75 millimeters in length. The fiber themselves may be specially preparedby techniques well known in the art or may be purchased commercially.

As needed, the cut ceramic fibers are cleaned by the appropriate methodsto remove any sizing or other potential contaminants which may be on thefiber surface. These cleaning methods include heating the fiberssufficiently to drive off the sizing material or washing the fibers inthe appropriate organic or inorganic solvents followed by drying thefibers.

Once cleaned, the cut fibers are dispersed and intimately placed incontact with carbon particles. The dispersion can take place by any wellknown means such as shaking the fibers and particles in air or in asolvent. When a solvent is used, the solvent is removed and thefiber-carbon particle mixture dried before further processing isapplied.

The carbon particles used in the processes of this invention should berelatively pure carbon having a particle size sufficient to create goodcontact with the dispersed fibers. Preferably, the carbon should have aparticle size averaging less than 20 um. Most preferably the averageparticle size should be less than 5 um.

The fibers dispersed in carbon are placed in a suitable vessel andannealed in the presence of a gas capable of reacting with the productsand byproducts formed during the heating of the oligomeric andpolymeric-derived silicon carbide fibers in the presence of the carbonparticles. Preferably, the gas is nitrogen gas although other gasescapable of reacting with the products and byproducts of the heatingprocess are believed to be equivalent to nitrogen.

The dispersed fibers are heated to a temperature above 800° C. andmaintained at that temperature for sufficient time for the fibers to bemodified. Preferably, the fibers are heated in an annealing-type processfrom room temperature to a treatment temperature between 800° C. and1800° C. and that temperature is maintained for a time sufficient forthe fibers to be modified and then the fibers are cooled to roomtemperature. Most preferably, the fibers are heated to a temperatureabove 1100° C. and below 1600° C. The time necessary for the fibers tointeract and be strengthened at the elevated temperature will vary fromfiber to fiber under the different interactive gases. It is preferredthat the fibers be treated for at least one hour and not more than 3hours to provide sufficient time for the fibers to be modified whilemaintaining sufficient strength.

The invention may be better understood by a reference to a specificexample.

EXAMPLE

A commercially available, titanium modified, polymer-derived siliconcarbide fiber is cut into 75 mm sections. The fiber sold under the tradename Tyranno fiber is TRN-401, Lot 82D514 purchased from UBE Industries,American, Inc., New York. The cut fiber pieces are heated to 350° C. inan oven to remove the sizing. The cleaned fibers are then ultrasonicallydispersed in hexanes together with carbon particles. The carbon powderis commerically available from Consolidated Astronautics, Inc. and is99.9% pure, with a particle size average of 2×10⁻⁶ meter, (2 micron) Lot115, Hauppauge, N.Y. The hexanes-fiber-carbon mixture forms a slurry.After dispersion, the hexanes solvent is removed and the fiber-carbondispersion is air dried. Then the carbon-fiber dispersion is placed in acovered aluminum oxide crucible. The crucible is placed in a graphitefurnace which is supplied with flowing nitrogen gas. The furnace isheated in flowing nitrogen to 1600° C. and maintained at thattemperature for 3 hours. The furnace is then cooled, the fibers areremoved from the oven and are separated by known techniques from thecarbon particle dispersion. These known techniques can includeseparating the fibers by hand or any of the methods used to separatewheat from chaff.

Fibers treated by the method of this invention were compared by scanningelectron microscope (SEM), transmission electron microscope (TEM), X-raydiffraction (XRD) and optical microscopy. The tensile strength oftreated and untreated fiber was determined in an apparatus which ismodeled after the ASTM standard D3379-75/10/ using a 12.5 mm gaugelength and a loading time of approximately 10 seconds. Because theloading closely approximates dead weight loading, traces of the fibercannot be located after testing so it was not possible to determine flaworigins. Cross sections of the fiber were prepared for TEM examinationby ion thinning fibers which are imbedded in an epoxy.

As seen in FIG. 1, the typical silicon carbide fiber before treatmentdoes not have a distinct microstructure or crystalline grain structure.This indicates the fiber is either amorphous or the crystallitestructure is too fine to be seen. X-ray diffraction and TEM analysisindicates that these fibers are microcrystalline, consisting primarilyof silicon carbide, with a crystallite size less than one nanometer(4×10⁻⁸ inches). A tensile strength test, which is a test where fiber isbroken by pulling it along its length, indicates the as-received-fiberhas a strength averaging 3.5 GPa (500,000 lbs./in²). For comparison,typical commercial ceramics average about 10 MPa (15,000 lbs per sq.in.) and the normal aluminum alloys used in aircraft have strengths ofabout 300-500 MPa (45,000-70,000 lbs. per sq. in.).

SEM examination of fracture surfaces of treated fibers compared tountreated fibers show differences between the two. Fibers which were notpacked in carbon had surfaces which were relatively smooth. The fracturesurfaces of carbon packed fibers had a granular or pebbly surface withrounded 50-100 nm features. In addition, there were changes within thecore of the fiber.

The core of the heated fiber changed from amorphous to a more mottledappearance with 1-2 nm silicon carbide crystals being formed. Thesemicrostructural changes along with the evolution of 30 nm voids indicatethat, during heating, atomic mobility is permitting material to proceedin the direction of an equilibrium phase assemblage. The core structureof treated and untreated fiber does not vary greatly but the surfacestructure does as described.

The fracture surfaces of the carbon packed fibers (FIG. 2) which aregranular or pebbly with rounded 50-100 nm features on the outer surfaceresemble a dense, sintered ceramic body. The outer approximately 0.5 umthick exterior shell or zone was found to consist of alpha siliconcarbide as opposed to the beta silicon carbide cystallites in the fiberinterior (FIG. 2B, C).

The as-received fibers lose strength dropping from 3.5 GPa to 1.2 GPa,when heat treated without carbon for 3 hours at 1600oC in nitrogen.Fibers undergoing carbon packing with an identical heat treatment cyclehave a smaller loss, dropping from 3.5 GPa to 1.9-2.5 GPa.

For comparison, tests of carbon packing of the oligomeric and/orpolymeric derived silicon carbide fibers were tested by packing thefibers in carbon and annealing the fibers with carbon particles under anargon atmosphere. The fibers produced were brittle and it was impossibleto measure tensile strength. No difference is found between carbonpacked and unpacked samples of the oligomeric or polymeric derivedsilicon carbide fibers annealed in an argon atmosphere. TEMmicrostructural examination shows that the microstructures are virtuallyidentical with no substantial crystal growth evident.

The carbon and the gas atmosphere interact to form the improved modifiedfiber. A comparison of the tensile strength versus temperature forfibers treated with nitrogen and carbon versus fibers annealed innitrogen only or an argon and carbon environment are shown in FIG. 3.

Although not essential to this invention, an understanding of thebelieved mechanism may help to understand the scope of the presentinvention. The method of fiber synthesis used to date results in fiberswhich contain excess of unreacted oxygen, carbon, silicon, and/ornitrogen.

Prior art untreated fibers also contain unstable microstructures. Thefibers are either glassy (amorphous or non-crystalline) ormicrocrystalline (composed of extremely small crystallites). Hightemperature exposure tends to convert the glassy material to acrystalline of microcrystalline material. Both effects are detrimentalto the strength of the fibers.

Heating the fibers also tends to cause chemical reactions andoutgassing. The outgassing typically takes the form of expellingnitrogen, carbon monoxide, or silicon monoxide vapor. Chemical reactionstake place both within the structure of the fiber and at its surface toproduce side products in the form of other compounds.

It is assumed that the strength degradation of fibers heated in nitrogenatmosphere without carbon present is partially caused by the formationof silicon nitride crystals at or in the surface of the fiber. Thesecrystals are formed by the reaction at the fiber surface of siliconmonoxide gas diffusing out of the fibers with nitrogen in thesurrounding atmosphere. Thermodynamic calculations indicate that thisreaction is a favored reaction, especially since the oxygen produced bythe reaction is continuously removed by combination with graphiteheaters in the furnace itself.

These silicon nitride crystals are assumed to act as flaws or weak spotscausing the fibers to fail when small loads are later applied. Suchfibers are also expected not to toughen a ceramic matrix because thefibers would tend to be locked into the matrix by the crystal structureand the fiber would break before it could slide and provide a resilienttough ceramic.

When carbon is present, the reaction goes forward in a similar mannerwith silicon monoxide gas reacting with nitrogen gas at the fibersurface to form silicon nitride. However, once silicon nitride forms, itfurther reacts with the carbon present to form a silicon carbide layeron the fiber and evolve nitrogen gas. The crystal structure of thissilicon carbide coating layer or zone, which can be seen in FIG. 2, ismuch smaller than the silicon nitride crystals and would not causeserious flaws.

It is expected that other gases, such as carbon monoxide gas, couldfunction as the interactive gas. In that case the carbon monoxide wouldinteract with the silicon monoxide being exuded at the fiber surface toagain form the silicon carbide and evolve oxygen. The chemical reactionmechanism actually occuring is complex and the suppositions made withrespect to this mechanism should not be used to limit the invention butshould be used to show the full scope and application of the invention.

Obviously, numerous additional modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practiced otherwise as is specifically describedherein.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A thermochemically treated surface modifiedoligomeric- or polymeric derived silicon carbide fiber having increasedability to retain physical and chemical properties after exposure toelevated temperatures so as to be more suitable for use in a compositematrix comprising a silicon carbide fiber derived from the groupconsisting of organosilicon oligomeric and polymeric precursors annealedat a temperature between 800° C. and 1800° C. in the presence of a gascapable of reacting with the products and byproducts formed during theheating of the oligomeric- or polymeric-derived silicon carbide fiberand in the presence of carbon particles to provide an outer zone ofsubstantially crystalline α-SiC upon a core of microcrystalline β-SiC.2. A fiber according to claim 1 wherein the gas capable of reacting withthe products and byproducts of the heated silicon carbide fibercomprises nitrogen gas.
 3. A fiber according to claim 2 wherein thetemperature is between 1100° C. and 1600° C.
 4. A fiber according toclaim 2 wherein the carbon particles are less than 20 microns in size.5. A fiber according to claim 4 wherein the temperature is below 1600°C.
 6. A fiber according to claim 5 wherein the fiber contains titanium.7. A fiber according to claim 1 wherein the gas capable of reacting withthe products and byproducts of the heated silicon carbide fibercomprises carbon monoxide.
 8. A silicon carbide fiber derived from thegroup consisting of organosilicon oligomeric and polymeric precursorsand having increased ability to retain physical and chemical propertiesafter exposure to temperatures so as to be more suitable for use in acomposite matrix comprising an outer zone of substantially crystallinealpha silicon carbide and a core of microcrystalline beta siliconcarbide.
 9. A silicon carbide fiber according to claim 10 wherein thecore of microcrystalline beta silicon carbide further comprises a dopingagent.
 10. A silicon carbide fiber according to claim 13 wherein thedoping agent comprises titanium.